Climate warming is expected to induce forest expansion into the tundra of today. However, surprisingly little is known about the forests that can be expected in such locations. How vast can these forests become, and how dense? A recent study by Asselin and Payette in the journal Ecography uses a novel approach to estimate the density of the "palaeo-forests" that once covered the tundra. By using soil charcoal fragments, the authors convincingly show that the treeless hilltops at the northernmost limit of trees in northern Québec had previously been dominated by black spruce and had switched to dwarf birch-dominated tundra after a fire, about 1000 years ago. The identification of charcoal remains using wood anatomy characters was instrumental in their documentation of this vegetation change, allowing the authors to provide evidence for the presence or absence of a number of species and estimates of their cover. This work provides a major step towards understanding the likely trajectory of vegetation change under climate warming, by analogy with vegetation composition under a past, warmer climate.